34 THE FRESH-WATER FISHES OF EUROPE. 



The head is characterised by the broad and deep pits which carry the 

 system of mucus-canals of the head, which are covered over only by the ex- 

 ternal skin. Many pits lie between and in front of the eyes ; one row is 

 contained in the curve of the sub-orbital bones. A bow-shaped line extends 

 along the cheek, but the most remarkable branch extends from the pre-oper- 

 culum to the lower jaw. This branch of the cephalic canal is covered by the 

 spines from the operculum. The pre-operculum is rounded posteriorly and 

 inferiorly, and is armed at its border with ten or a dozen spines, which diverge. 

 The lower spines are larger and farther apart than the upper ones, which are 

 smaller and serrated. The operculum ends in a short, sharp spine, which is 

 covered by a small flap of skin. The sub-operculum is large, and slightly 



denticulated on its anterior border. Villiform teeth occur on the pre-maxillary, 

 the maxillary, and pharyngeal bones. Small teeth are also irregularly dis- 

 tributed on the tubercles of the branchial arches. Some small teeth have been 

 observed on the palatine bones in specimens from the River March, in Austria. 



The dorsal fin occupies nearly the entire length of the back, and com- 

 mences above the opercular spine and pectoral fin. It usually contains fourteen 

 spinous rays, of which the fourth to sixth are the longest, and the succeeding 

 rays decrease in length to the last spinous ray, so that the outline of this part 

 of the fin is a curve. There are eleven to fourteen soft rays in the hinder 

 part of the fin. 



The anal fin is opposite to the membranous part of the dorsal, but does not 

 reach quite so far back. The pectoral and ventral fins are of similar length 

 the pectorals are oval, and contain thirteen rays ; the ventral fin has one thick 

 spiny ray and five soft rays. The caudal fin is moderately notched, and is 

 formed of seventeen rays. The anal aperture is behind the middle of the 

 length of the fish. 



